1. Field of the Invention
The present invention is directed to silicone rubber compositions and, more particularly, to silicone rubber compositions incorporating silicon-treated carbon blacks and processes for making the same.
2. Description of the Related Art
Silicone rubber compositions are used in a wide variety of applications due to their superior and unique properties relative to other organic elastomers over a wide temperature range. For example, silicone rubber compositions provide superior heat resistance compared to other elastomers. Moreover, silicone rubbers have low electrical conductivity and, therefore, excellent insulating properties, particularly under wet conditions. In addition, silicone rubbers have good chemical, fuel, and oil resistance, and are weatherable and resistant to ozone attack. It is believed that these properties of silicone rubber compositions are due to the unusual molecular structure of the polymer, which typically consists of dimethyl siloxane units.
A typical silicone rubber composition contains a silicone polymer, reinforcing and extending fillers, processing aids or softeners to plasticize and retard crepe hardening, additives, colorants, and one or more curing agents. The silicone rubber composition may be compounded by methods known to those skilled in the art utilizing conventional process equipment, such as extruders, Dough mixers, Banbury mixers and two-roll mills.
Notwithstanding the above-noted properties, pure silicone rubber polymers, or polydimethyl siloxanes (also called "gums"), are weak elastomers. As a result, although silicone polymers may be used alone, rubber fabricators often compound silicone rubber elastomers from silicone gums or bases that contain reinforcing and/or extending fillers. In addition, these fillers are added to silicone rubber to increase their hardness and reduce their stickiness.
Various fine particle silicas are commonly used as rheology control agents to provide easy application and retention of an extruded shape during curing. Fumed silica also is the most commonly used reinforcing filler, providing superior reinforcement, and increased hardness, tensile, and elongation in silicone rubber elastomers. Typically, the reinforcing silica is incorporated into the silicone polymer prior to vulcanization. The high purity of fumed silica provides the silicone rubber composition with excellent physical and insulating properties. Depending on the desired properties and the end use application, between about 5.0 and about 40.0 percent, by weight, fumed silica is typically used in silicone rubber compositions.
In addition to fumed silica, precipitated silicas may be used as reinforcing fillers to provide silicone rubber compositions with moderately high reinforcement and improved physical properties such as tensile strength and elongation. In addition to fumed and precipitated silicas, other materials are used as semi-reinforcing or extending fillers, as well as processing aids, additives, and colorants, to obtain an optimum balance of physical properties, costs, and processability. For example, zinc oxide is used as a colorant and as a plasticizer, imparting heat conductivity, tack and adhesive properties to a rubber composition. Similarly, red iron oxide can be used as colorant and as a stabilizer against heat aging. Carbon black provides silicone rubber compositions with electrical conductivity, and is an additive and colorant that is known to provide moderate reinforcement.
Processing aids are often add to highly reinforcing silica fillers to provide a softening or plasticizing effect. In addition, the shelf life of the compounded rubber is increased by slowing the crepe-hardening effect in the silicone rubber that occurs as a result of bonding of the polymer and the reinforcing filler.
Finally, typical silicone rubber compositions include one or more curing agents including peroxides. Among the variety of peroxide curing agents, not one can be used for all types of silicone polymers. For example, aroyl peroxides may be considered general-purpose curing agents in that they will cure both non-vinyl and vinyl-containing silicone polymers. However, these agents are not suitable in all fabrication procedures. In addition, vinyl-specific peroxides such as dicumyl peroxide, and others, may also be used to vulcanize silicone rubber.
As noted above, in addition to being used as an additive and colorant, carbon black may also be used to provide moderate reinforcement in silicone rubber compositions. However, carbon black is known to inhibit cure with non-vinyl and vinyl-containing silicone polymers, in addition to reducing the thermal stability of silicone rubber compositions. These disadvantages tend to limit the use of carbon black to applications requiring electrically conductive or semi-conductive rubbers and/or as a colorant.
In applications where the high to moderately-high reinforcement provided by fumed and precipitated silicas is not required in silicone rubber applications, a need remains for alternative fillers and additives that provide the desired level of reinforcement and physical properties, such as hardness, tensile strength and elongation. In addition, alternative fillers and additives are desired to reduce compounding costs and enhance processability of silicone rubber compositions.